1. Field of the Invention
The present invention relates to a display apparatus equipped with optical elements, pixel by pixel, which perform an optical operation according to a current value, particularly, optical elements which emit light with a brightness according to the current value, and a drive method for the display apparatus.
2. Description of the Related Art
In general, there are two types of display apparatuses: a passive driving type and an active matrix driving type which has switching transistors provided pixel by pixel. In a liquid crystal display of the active matrix driving type, as shown in FIG. 11, a liquid crystal element 501 having a liquid crystal which also serves as a capacitor, and a transistor 502 which serves as a switching element are provided for each pixel. In the active matrix driving type, when a data driver applies a voltage for controlling the transmittivity of the liquid crystal to a signal line 504 while a scan line 503 is selected in a select period as a pulse signal is input to the scan line 503 by a scan driver, a voltage is applied to the liquid crystal element 501 via the transistor 502. In the liquid crystal element 501, liquid crystal molecules are aligned in the direction according to the applied voltage to adequately change the transmittivity of light which passes the liquid crystal element 501. Even if the transistor 502 is turned off in a non-select period following the select period, however, the liquid crystal element 501 serves as a capacitor to hold charges according to the invention the voltage value in the allowable range until the next select period, the alignment direction of the liquid crystal molecules is maintained over that period. As apparent from the above, the liquid crystal display is a voltage-controlled type display apparatus in which a voltage is newly written in the select period in such a way that the light transmittivity becomes that of the liquid crystal element 501 and which provides arbitrary gradation expression according to the voltage value.
Unlike the liquid crystal display, a display apparatus that uses an organic EL (Electroluminescence) element which is a self light-emitting element requires no backlight and is optimal for ensuring a flatter display and is free of the restriction on the angle of visibility that the liquid crystal display has. In this respect, this organic EL type display apparatus is promising as the next generation of the display apparatus and its practical usage is greatly expected.
From the view points of high luminance or brightness, high contrast and high definition, it is particularly desirable that the organic EL display, like the liquid crystal display, should be of the active matrix driving type. While a passive driving organic EL display requires that the current flowing in the select period should be increased, an active matrix driving type is provided, for each pixel, with an element for holding the voltage applied across both ends of an organic EL element in order to allow the organic EL element to keep emitting light with a predetermined brightness for light emission even in a non-select period, thus ensuring continuous flow of the current even in the non-select period. Therefore, the value of the current that flows in the organic EL element per unit time in the select period and non-select period could be small. Because the organic EL element has a very small capacitance as a capacitor, however, the organic EL element, when merely provided in the pixel circuit as shown in FIG. 11 in place of the liquid crystal element 501, cannot keep emitting light until the non-select period.
As one example of solutions to this shortcoming, as shown in FIG. 12, an active matrix driving type organic EL display is provided, for each pixel, with an organic EL element 601 which emits light with a brightness proportional to the value of the current flowing inside, a transistor 602 which serves as a switching element and a transistor 605 which allows the drive current according to the gate voltage applied by the transistor 602 to flow in the organic EL element 601. In this display, when a signal voltage for letting the drive current having a predetermined current value flow to the transistor 605 is applied to a signal line 604 by the data driver while a pixel connected to a scan line 603 is selected as a pulse signal is input to the scan line 603 in the select period by the scan driver, the voltage is applied to the gate electrode of the transistor 605 and brightness data is written in the gate electrode of the transistor 605. This sets the transistor 605 on so that the drive current with the gradation according to the invention the value of the voltage applied to the gate electrode flows into the organic EL element 601 via the transistor 605 from a power supply and the organic EL element 601 emits light with the brightness according to the current value. In the non-select period following the select period, even when the transistor 602 is turned off, charges according to the gate-source voltage of the transistor 5605 are kept stored by the parasitic capacitor or the like between the gate and source of the transistor 605, thus keeping supplying the drive current to the organic EL element 601. In short, the drive current is specifically controlled by the value of the gate voltage of the transistor which is output in the select period so that the organic EL element 601 is allowed to emit light with a predetermined gradation brightnesses.
Generally, as the channel resistance of a transistor depends on the ambient temperature or varies over a long usage, the gate threshold voltage changes with time or varies. Even if the value of the current flowing in the organic EL element 601 is changed by changing the voltage to be applied to the gate electrode of the transistor 605, i.e., even if the brightness of the organic EL element 601 is changed changing the voltage to be applied to the gate electrode of the transistor 605, therefore, it is difficult to specifically write the value of the current flowing in the organic EL element 601 with the gate voltage of the transistor 605.
In this respect, a study has been made on a scheme for controlling the brightness based on the current value, not based on the level of the voltage to be applied to the transistor. That is, a current-writing system to directly supply the value of the current flowing in an organic EL element to a signal line, not a voltage-writing system which supplies the level of the gate voltage to a signal line, is adapted to the active matrix driving for an organic EL display.
In the current-writing display, the value of the current that flows per unit time should be made smaller. In case where the current-writing system is adapted to an active matrix driving organic EL display, therefore, an organic EL display provided with a transistor which reduces the value of the current flowing into the organic EL element with respect to the output current from the driver as shown in FIG. 13 has been proposed. For each pixel, the organic EL display in FIG. 13 is provided with an organic EL element 701, an N type MOS transistor 702 and a P type MOS transistor 707 which serve as switching elements, a P type MOS transistor 706 which converts the value of a write current to a gate voltage, a capacitor 709 which holds charges according to the gate voltage of the transistor 706 and a P type MOS transistor 705 which lets a drive current with the gradation according to the held charges flow into the organic EL element 701.
In this display, as the write current is allowed to flow in a signal line 704 by the data driver while the transistors 707 and 702 are selected by signals output onto a first scan line 708 and a second scan line 703, a voltage is produced between the gate and source of the transistor 706 and the current according to the channel characteristic of the transistor 705 and the channel characteristic of the transistor 706 flows into the transistor 705 and the organic EL element 701.
The organic EL display in FIG. 13 should have however been provided with a current mirror circuit comprised of the transistors 705 and 706 for each pixel, and the increase in the number of pixels made the production yield of displays lower due to defects in the transistors 705 and 706. When the transistors 705 and 706 and the organic EL element 701 are laid out two-dimensionally, the ratio of the area of the non-luminous area, such as a transistor, of the pixel to the area of the luminous area increases, so that a burden is put on the organic EL element 701 to ensure bright display. To supply the current to an element, such as an organic EL element, which emits light on a minute current, in particular, the transistor 706 should have been designed larger with respect to the transistor 705, resulting in a considerable increase in the ratio of the non-luminous area to the luminous area. There had occurred an additional problem such that the probability of occurrence of an in-plane variation in the current characteristics of the transistors 705 and 706 which serve as a current mirror circuit would become higher in proportional to an increase in the number of pixels. Further, while the transistor 702 is an N channel transistor, the other transistors 705 to 707 are P channel transistors. This necessitates that different channel types of transistors should be fabricated, thus lowering the throughput.
One advantage of the invention lies in that the display can emit light on a current with the adequate current value and the ratio of the luminous area to the non-luminous area is high, and another advantage lies in that the display has a structure which is easy to manufacture.